Ever since it has become apparent that the incidence of burglaries in private homes, especially burglaries and thefts of personal appliances such as televisions and VCRs, a multitude of alarms and alarm systems have been developed to thwart this problem. Alarms for protection of personal property from theft, for example in such places as museums and retail stores, have generally included some sort of plunger for sensing the removal of the object. See, for example, U.S. Pat. No. 4,274,088. The problem with this type of alarm as it relates to appliances in the home is that a thief can easily deactivate the alarm by keeping the plunger depressed while removing the appliance.
Another example is British patent No. 1,409,608 which discloses a loop alarm in which an electric circuit connected to an audible or visual alarm is located in a base on which articles are to be supported for display. A number of apertures are provided in the base within each which is located a light sensitive electrical component connected to the electric circuit. Articles on display are positioned over the light sensitive electrical components but if they are removed light is emitted to the component and actuates the alarm, Another British patent No. 1,600,798 provides an anti-theft device comprising an alarm module capable of producing a detectable alarm signal using a single electrical cable connected to the module to form a loop.
In 1974 U.S. Pat. No. 3,836,901 was issued to Matto et al, for a circuit and apparatus adapted to be mounted entirely or in part within the divece to be protected including an alarm circuit loop and a control loop with the state of the control loop being determined by whether the line cord of the protected device is in or out of the wall socket. The control loop can thus disable or enable the alarm circuit depending on the position of the power cord. However, actual operation of the alarm circuit requires both a change in the state of the control circuit due to the state of the line card and a change in the alarm circuit due to movement of the protected device. This type of arrangement does not allow or provide a mechanism to prevent the alarm from engaging as a result of momentary lapses in power to the appliance. This prior art device is operated utilizing a transformer.
A transformer is a rather heavy, expensive item which has unduly increased and made impractical the prior art line cord responsive alarm. The additional drawback of prior art line sensitive alarms is that they can not be easily adapted to present production lines. Moreover, the transformer alarm device has to be interconnected to the appliance and power input system.